There is a continued increase in the requirement for highly durable cutting tools in response to the demand for and manufacture of more exotic, sophisticated high strength, structural materials used particularly within the aerospace and automotive industries. These materials bring substantial benefits to these industries due to their very high strength to low weight ratio.
For example, highly used carbon-based composite, workpiece materials laminated with aluminum alloy sheeting, are so abrasive that unprotected, standard cutting tools are capable of cutting a relatively small portion only of the workpiece material. Being dependent upon the composition of the laminates, the cutting process has to cope with different cutting actions in the aluminium and the hard and abrasive synthetic component. This is further complicated by the orientation of the top and bottom layers when “through” drilling the composite materials. A composite with an aluminium top and bottom layer is easier to machine than a material with a synthetic top and bottom layer due to the tearing of the fibre strands on entry and especially at break through. The cutting edges of such tools are blunted very rapidly after, for example only two drilling operations, whereby the tool is no longer suitable for use and must be discarded.
An additional class of materials that is very difficult to machine are commonly known as ‘sticky alloys’ such as nickel and aluminum based alloys. When a cutting tool is employed to mechanically cut such materials, a significant temperature rise is observed in the region of contact between the cutting tool and alloy based material. This arises as a result of the mechanical energy required to overcome frictional resistance as sliding at the contact interface occurs at the micron level. This increase in temperature can be substantial and can result in localized changes in the material properties including increased chemical reactivity and in particular chemical interaction between the workpiece material and the cutting tool or any coating applied to the cutting tool.
A further class of material that presents machining problems includes stainless steel and titanium based alloys. Both these types of alloy exhibit low thermal conductivity resulting in intensive heat generation at the cutting region. In particular the temperature of a cutting edge of a tool is so high that it is common to detect micro welding or material transfer due to the intensified chemical interaction between the workpiece material and the cutting tool or coating formed on the tool. Consequently, it is a common problem for the geometry of the cutting tool to alter, following only small or moderate periods of cutting of these types of alloy, thereby deteriorating the cutting performance.
In an attempt to address the above problems, many cutting tool coatings have been developed. The main, desirable properties of such coatings include a low friction coefficient and very high wear resistance.
One such coating that attempts to address the above problems is disclosed in EP 0870565. An edge portion of the cutting tool is coated with at least one layer of a film of composition (Ti(1-x)Alx)(Ny C(1-y)) where 0.2≦x≦0.85 and 0.25≦y≦1.0.
US 2003/0148145 discloses a hard film exhibiting high wear resistance, with composition of (Alb,[Cr1-αVα]c(C1-dNd), satisfying the condition of 0.5≦b≦0.8, 0.2≦c≦0.5, b+c=1, 0.05≦α≦0.95, 0.5≦d≦1 (where b and c each represent atomic ratio of Al and Cr+V, and d denotes atomic ratio of N, α denotes atomic ratio of V), or with composition of (Mα,Alb,[Cr1-αVα]c(C1-dNd), wherein M is at least one element selected from Ti, Nb, W, Ta and Mo and satisfying the condition of 0.02≦α≦0.3, 0.5≦b≦0.8, 0.05≦c, α+b+c=1, 0.5≦d≦1, 0≦α≦1 (where α represents atomic ratio M). The layered coating disclosed includes two or more layers of hard films laminated together and different from each other. Alternatively, the coating is formed as a single layer. A method of forming the wear resistance film is also described.
EP 0846784 discloses a coated tool and method of manufacturing the same. The coated tool comprises a base material and a wear resistant coating film formed on the base material. The composition of the wear resistant coating film is expressed as (Tix,Aly,Vz)(Cu,Nv,Ow). Relations x+y+z=1, u+v+w=1, 0.2<x≦1 and 0≦y<0.8, 0.02≦z<0.6, 0≦u<0.7, 0.3<v≦1 and 0≦w<0.5 hold between x, y, z, u, v and w. The thickness if the wear resistant coating film is stated as at least 0.5 μm and not more than 15 μm. The coated cutting tool comprises a case material consisting of cemented carbide and a wear resistant coating film formed on the surface of the base material. The outermost surface of the wear resistant coating film is coated with a low melting point oxide, containing vanadium oxide, having a melting point of not more than 1000° C.
EP 0999290 discloses a wear resistant hard coating and a cutting tool coated with the same. The hard coating includes an adhesion reinforcing layer formed on a surface of the tool; and a second layer on the adhesion reinforcing layer and having a composition represented by: (AlpTiqVr)(NuCv) where 0≦p≦0.75, 0.20≦q≦0.75, 0.10≦r≦0.75, p+q+r=1, 0.6≦u≦1, and u+v=1. In order to further enhance the adhesion of the hard coating to the cutting tool an intermediate layer may be formed between the first and second layers.
JP 4221057 discloses a wear resistant coating film of thickness 0.8-10 μm and having a chemical composition represented by (VxTi1-x)(NyC1-y) where 0.25≦x≦0.75 and 0.6≦y=1 formed on the surface of a substrate using an arc discharge process.
Nanoscale multilayer superlattice PVD coatings exhibit high hardness, (HP>40 GPa), wear resistance, and excellent protection against corrosion depending upon the choice of material partners. The terms superlattice, within the context of coatings, refers to a high-hardness coating having a modulating layered structure of two or more materials with nanometer-layer thickness dimensions. Superlattice structures are qualified by the distance between each successive pair of layers which is typically know as the ‘bilayer repeat period’. Various material coating combinations based on TiN and TiAlN are disclosed in Surface and Coating Technology 133-134 (2000) 166-175 and Surface Engineering 2001 vol. 17 no. 1 15-27. These papers describe the properties of coatings dedicated to high temperature performance, tribological applications and combined wear and corrosion resistance.
Moreover, using appropriate PVD equipment super-hard coatings can be deposited in a reproducible manner allowing great variety of material compositions. By selecting the appropriate layer composition, wear resistant coatings specialised for high temperature, low friction and corrosion resistant applications are possible.
However, such coatings whilst addressing the problems of wear resistance are still susceptible to micro welding and workpiece material pick-up due to the intensified chemical interaction between the coating and the workpiece material resulting from friction and hence heat generation at the tool-workpiece interface.
What is required therefore is a coating suitable for a substrate which addresses the above identified problems.